Electrically erasable and programmable non-volatile semiconductor memory with automatic write-verify controller

ABSTRACT

A NAND-cell type EEPROM includes an array of memory cells connected to bit lines. Each cell includes one transistor with a floating gate and a control gate electrode, wherein electrons are tunneled to or from the floating gate to write a data thereinto. A sense/latch circuit is connected to the bit lines, and selectively performs a sense operation and a latch operation of the write data. A program controller is provided for writing the data into a selected memory cell in a designated area, and for reading the data written in the selected cell to verify whether or not its resultant threshold voltage falls within an allowable range. If it is insufficient, the data is rewritten thereinto. A rewrite-data setting section is provided for performing a logic operation with respect to a read data from the selected cell and the write data being latched in the sense/latch circuit, and for updating automatically a rewrite data being stored in the sense/latch circuit with respect to every bit line in accordance with the actual write state as being verified. The sense/latch circuit includes a CMOS flip-flop circuit, which acts as a data-latch at the beginning of the verify operation, and serves as a sense amplifier once after it is reset.

This is a continuation of application Ser. No. 07/948,002, filed on Sep.21, 1992 U.S. Pat. No. 5,357,468.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrically erasable andprogrammable non-volatile semiconductor memory devices, and moreparticularly to an electrically erasable and programmable read onlymemory having an array of memory cells each of which essentiallyconsists of one transistor.

2. Description of the Related Art

Recently, NAND-cell type EEPROMs have been developed as one of highlyintegrated electrically erasable and programmable read-only memory(EEPROM) devices. With such type of EEPROMs, an array of rows andcolumns of memory cells are divided into a plurality of cell sectionscoupled to parallel bit lines. Each cell section includes apredetermined number of memory cell transistors that are connected inseries to one another with each of intermediate active layers acting asthe source and drain of adjacent memory cell transistors. Each memorycell transistor may be a floating-gate metal oxide semiconductor fieldeffect transistor having an insulated gate (floating gate) for storingelectrical charge carriers therein and a control gate coupled to acorresponding word line.

The NAND cell array is arranged either in a P type silicon substrate orin a P type well region formed in an N type silicon substrate. A memorycell transistor positioned at the first end of each NAND cell sectionhas a drain coupled to a corresponding bit line by way of a first selecttransistor. The source of another memory cell transistor that is at theopposite end of the NAND cell section is coupled to a common sourcevoltage (a reference potential wiring line) through a second selecttransistor. The control gate electrodes of NAND memory cell transistorsare connected to each other along the row direction to constitute wordlines on the substrate.

The operation of the conventional NAND-cell type EEPROM arranged asdescribed above is as follows. A data write for a selected cell sectionis carried out in such a manner that the memory cell transistorsincluded therein are sequentially subjected to write operations with amemory cell transistor that is most distant from the memory celltransistor connected through the first select transistor to acorresponding bit line associated therewith (i.e., the memory celltransistor coupled to the common source potential through the secondselect transistor) being as a starting cell transistor. A boosted highvoltage Vpp (20 volts, for example) is applied to the control gate of amemory cell transistor being presently selected for write. Anintermediate voltage VppM (such as 10 volts) is applied to the selectgate and the control gate(s) of one or those of the memory celltransistors which are positioned between the selected cell transistorand the first select transistor, thereby to render these transistorsconductive. A zero-volt voltage or the intermediate voltage VppM isapplied to the corresponding bit line in accordance with the logic valueof the write data.

when the zero-volt voltage is applied to the bit line, a resultantpotential thereon is transferred to the drain of the presently selectedmemory cell transistor through the transistors being renderedconductive. Electrons are thus injected from the drain into the floatinggate of the selected cell transistor. The threshold voltage of it isthus shifted positively. This positive shift condition is defined as alogic "1" storage state. Alternatively, when the intermediate voltageVppM is applied to the bit line, the injection of electrons does nottake place, so that the selected cell transistor is kept unchanged inits threshold voltage. This condition is defined as a logic storagestate.

A data erase is carried out so that all the memory cell transistorsincluded in the NAND-cell type EEPROM are erased at a time. Morespecifically, while the control gates and the first and second selectgates are set at the zero volts, (1) the bit lines and the common sourceline are rendered electrically floating, and (2) the high voltage Vpp isapplied to the P type substrate (or both the P type well region and theN type substrate). As a result, electrons accumulated in the floatinggates are released to the P type substrate (or to the P type wellregion) in all the memory cell transistors, causing the thresholdvoltages of them to be shifted negatively.

A data read is performed by detecting whether or not a current flows ina selected memory cell transistor while causing the control gate of theselected cell transistor to be at zero volts, and applying a powersupply voltage Vcc (5 volts, for example) to the control gates of theremaining memory cell transistors and the select gates.

As is apparent from the above explanation, according to the presentlyavailable NAND-cell type EEPROM, the non-selected memory celltransistors act as "transfer gates" that allow the write data to betransferred to or from the selected memory cell transistor during thewrite and read periods. From this viewpoint, the following inevitablerestriction is put on the setting of an allowable range of the thresholdvoltage of the once-written memory cell transistor: The thresholdvoltage of a memory cell transistor being written with a logic "1"should range from 0.5 to 3.5 volts. By taking into account thedeterioration with age in the threshold voltage after the completion ofdata write, possible deviations in the manufacturing parameters of thememory cell transistors, a potential variation of the power supplyvoltage Vcc, and so on, it is required in practical applications topractice that the allowable variation range of the threshold voltage isso designed so as to be narrower than the above.

However, with a conventional write scheme using a fixed writingpotential and a fixed writing time with respect to all the memory celltransistors to cause them to be written under the same conditions, it isnot easy to force the threshold voltage variation after writing of alogic "1" to fall within the limited allowable range. For example, thememory cell transistors may vary in physical property due to somevariations that possibly occur in the manufacturing processes. Regardingthe write characteristic, this results in that easy-to-write cells anddifficult-to-write cells coexist in the NAND cell section. To facilitateto attain a successful programming for such cells different in writecharacteristic from one another, a specific "variable write-time/verify"programming architecture has been proposed which includes a process ofadjusting the writing time with respect to each memory cell according toits inherent write characteristic, and a process of checking orverifying the validity of the data once written into each each cell.Unfortunately, the conventional NAND-cell EEPROM still cannot take themaximum advantages out of the advanced programming architecture whilehaving the integration density higher than ever. The main reason forthis is that the EEPROM is conventionally required to include two extraflip-flop circuits that are arranged at the both ends of each bit lineto perform a data latch operation and a sense amplification operation.The number of such extra circuits will increase as the number of bitsincreases. The addition of the increased number of extra circuits leadsto an undesirable increase in the occupation area of the internalcircuitry of the EEPROM, causing its integration density to decrease.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new andimproved electrically erasable and programmable semiconductor memorydevice that can attain a higher integration density and a higherreliability.

It is another object of the present invention to provide a new andimproved electrically erasable and programmable semiconductor memorydevice that enables the threshold voltage variation of a once-writtenmemory cell to fall within a limited allowable range while maintainingan increased integration density of memory cells.

In accordance with the above objects, the present invention is drawn toa specific electrically erasable and programmable non-volatilesemiconductor memory device, which includes a semiconductive substrate,a plurality of bit lines on the substrate, and an array of rows andcolumns of memory cells connected to the bit lines on the substrate.Each of the memory cells includes a transistor having an insulatedcarrier storage layer and a control gate electrode insulatively disposedabove the carrier storage layer, wherein electrical charge carriers moveby the tunnel effect to or from the carrier storage layer for a datawrite. A sense/latch circuit is coupled to the bit lines, forselectively performing a sense operation and a data latch operation forlatching a write data. A program controller is provided for writing thewrite data into a selected memory cell or cells of the memory cellswhich are included in a designated area during a predetermined length oftime period, for reading the contents of the selected memory cell orcells to verify whether or not a resultant threshold voltage thereoffalls within a predetermined range, and for, if an insufficientlywritten memory cell transistor is found, writing the write data againinto the insufficiently written memory cell transistor. A data setter isprovided for performing a logic operation with respect to a read datacorresponding to the contents of the selected memory cell or cells andthe write data being latched in the sense/latch circuit means, and forautomatically updating a rewrite data being stored in the sense/latchcircuit means with respect to every bit line in accordance with theactual write state as being presently verified. The sense/latch circuitincludes a flip-flop circuit which remains as a latch circuit at thebeginning of the verify operation, and functions as a sense amplifiercircuit after being reset.

The foregoing and other objects, features and advantages of theinvention will become apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing schematically the overall arrangement of aNAND-cell type electrically erasable and programmable read-only memory(EEPROM) in accordance with one preferred embodiment of the invention.

FIG. 2A shows a plan view of one NAND cell section included in a memorycell array of FIG. 1, and FIG. 2B is a diagram showing the equivalentcircuit of the NAND cell section of the preceding figure.

FIGS. 3A and 3B illustrate two cross-sectional structures of the NANDcell section of FIGS. 2A and 2B along two intersecting lines A--A andB--B respectively.

FIG. 4 is a diagram showing the overall configuration of internal memorycells of the cell array of FIG. 1.

FIG. 5 is a diagram showing the internal arrangement of a bit-linecontrol circuit of FIG. 1.

FIG. 6 is a timing diagram showing potential variations occurring at themain components of the embodiment during a read operation with respectto the even-numbered columns of the embodiment, and FIG. 7 is a timingdiagram showing potential variations occurring at the main componentsduring a read operation with respect to the odd-numbered columns of theembodiment.

FIG. 8 is a timing diagram showing potential variations at the maincircuit components during write and verify-read operations of theembodiment, and FIG. 9 is a timing diagram showing potential variationsat the main circuit components during rewrite and verify-read operationsof the embodiment.

FIG. 10 is a diagram showing the circuit configuration of a modificationof the bit-line control circuit which is utilized in a second embodimentof the invention.

FIG. 11 is a timing diagram showing potential variations at the maincircuit components of the second embodiment, and FIG. 12 is a timingdiagram showing potential variations at the main components during writeand verify operations.

FIG. 13 is a diagram showing schematically the overall arrangement of aNAND-cell type EEPROM in accordance with a third embodiment of thepresent invention.

FIG. 14 is a diagram showing the internal circuit configuration of abit-line control circuit of FIG. 13.

FIG. 15 is a timing diagram showing potential variations at the maincircuit components of the third embodiment during a read operation withrespect to the columns of a first memory cell block of the embodiment;FIG. 16 is a timing diagram showing potential variations at the maincomponents during a read operation with respect to the columns of asecond memory cell block of the embodiment; FIG. 17 is a timing diagramshowing potential variations at the main components during a writeoperation; and FIG. 18 is a timing diagram showing potential variationsthereat during a verify operation of the third embodiment.

FIG. 19 is a diagram showing the arrangement of a memory cell matrixsection of a NOR-type EEPROM in accordance with a fourth embodiment ofthe invention.

FIG. 20 is a diagram showing the internal arrangement of a bit-linecontrol circuit used in the NOR-type EEPROM of FIG. 19.

FIG. 21 is a timing diagram showing potential variations at the maincircuit components of the fourth embodiment during write and verifyoperations thereof.

FIGS. 22 to 24 are diagrams showing three possible circuitconfigurations that are preferably used in the above-mentioned NAND-celltype EEPROMs.

FIG. 25A is a diagram showing an exemplary circuit configuration of adrive circuit for boosting the potentials of signals, which ispreferably employed in a case wherein the concept of the presentinvention is applied to an EEPROM operative under a decreased powersupply voltage therefor, and FIG. 25B is a timing diagram of theembodiment shown in the preceding figure.

FIG. 26A is a diagram showing another exemplary circuit arrangement ofthe drive circuit for boosting the potentials of signals, and FIGS. 26Band 26C are timing diagrams therefor.

FIG. 27 is a timing diagram showing potential variations at the maincircuit components of the EEPROM using the driver circuit shown in FIG.25A or 26A during a read operation with respect to the columns in afirst memory cell block thereof; FIG. 28 is a timing diagram showingpotential variations at the components during a read operation for thecolumns in a second memory cell block of the EEPROM; FIG. 29 is a timingdiagram showing potential variations at the components during a writeoperation; and FIGS. 30 and 31 are timing diagrams showing two possiblepotential variations thereat during a verify-read operation.

FIGS. 32A to 32H are diagrams showing several arrangements of a bit-linecontrol circuit that is possibly employed in the above-mentionedNAND-cell EEPROMs.

FIGS. 33A to 33H are diagrams showing several arrangements of a bit-linecontrol circuit that is possibly employed in the above-mentionedNOR-cell EEPROM.

FIGS. 34A and 34B illustrate flowcharts of the write and write-verifyoperations in the EEPROMs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, an electrically erasable and programmable readonly memory (EEPROM) of the NAND cell type in accordance with onepreferred embodiment of the present invention includes an array 1 ofmemory cells arranged in rows and columns. The memory cell array 1 isformed in a P type silicon substrate ("11" shown in FIGS. 3A and 3B).Memory cell array 1 may alternatively be arranged in a P type wellregion formed in an N type silicon substrate.

The memory cell array 1 is connected to a bit-line control circuit 2,which includes a sense amplifier circuit and a data latch circuit andwhich performs a write, a read, a rewrite and a verify operation formemory cell array 1. The bit-line control circuit 2 is connected to acolumn decoder 3. Column decoder 3 is coupled to an address buffer 4.Address buffer 4 supplies column decoder 3 with an address signal.Address buffer 4 is also connected to a row decoder 5 that is coupled tomemory cell array 1. Row decoder 5 potentially controls the controlgates and select gates of memory cell array 1.

The bit-line control circuit 2 is connected to a data input/output (I/O)buffer 6, and receives the output of the column decoder 3, which isresponsive to an address signal supplied by address buffer 4. Memorycell array 1 is coupled to a substrate potential control circuit 7,which potentially controls the P type silicon substrate (or the P typewell region in the N type silicon substrate).

The bit-line control circuit 2 essentially consists of a complementarymetal oxide semiconductor (CMOS) flip-flop circuitry. Bit-line controlcircuit 2 is the circuit that carries out a latch operation forpotentially latching a logic data to be written into a memory cell beingpresently selected, a sense operation for sensing a bit line potentialto read a data, a sense operation for a verify-read after the writeoperation, and a latch operation for potentially latching a data to berewritten (rewrite data) when any "insufficient write" is detected.

The memory cell array 1 of FIG. 1 is divided into a plurality of cellsections, each of which includes a predetermined number of floating gatetunneling metal oxide semiconductor (FATMOS) field effect transistors Meach constituting one "cell." With the embodiment, these "memory celltransistors" are assumed for purposes of explanation to be eight FATMOStransistors M1, M2, M3, . . . , M8. As shown in FIG. 2B, the memory celltransistors M are connected in series with one another in such a mannerthat an intermediate active region serves as both a drain of a celltransistor and a source of another cell transistor neighboring thereto,thereby providing a "NAND cell" structure. The cell section of FIG. 2Awill be referred to as a "NAND cell section" hereinafter. The twodifferent cross-sectional structures of the NAND cell section are shownin FIGS. 3A and 3B respectively.

As shown in FIG. 3A, the series circuit of memory cell transistors M1 toM8 are formed in the top surface of P type substrate 11 (or P type wellregion not shown), which is surrounded by an element-separation oxidefilm 12. Memory cell transistors M1-M8 have floating gate electrodes14-1, 14-2, . . . , 14-8 electrically insulated by a dielectric film(gate insulation film) 13 from substrate 11, and control gate electrodes16-1, 16-2, . . . , 16-8 insulated by another dielectric film 15.Heavily-doped N (N+) type diffusion layers 19 are formed in substrate 11as the sources or drains of memory cell transistors M. Each ofintermediate diffusion layers except the both side ones serves as thesource and drain of adjacent memory cell transistors.

On the drain side of the series array of NAND cells, a first selecttransistor S1 is arranged which has two stacked gate gate layers 14-9,16-9 being electrically coupled with each other. A second selecttransistor S2 is provided on the source side of the NAND cell array tohave gate layers 14-10, 16-10 that are electrically coupled with eachother. Memory cell transistors M and the first and second selecttransistors S1, S2 are covered by a CVD dielectric film 17, on which ametallic wiring layer 18 is disposed to constitute a bit line BLi (i=0,1, 2, . . . ). As shown in FIG. 3A, bit line layer 18 is coupled,through a contact hole formed in CVD dielectric film 17, to onediffusion layer 19 acting as one of the current-carrying electrodes offirst select transistor S1.

The control gate electrodes 16 of each subarray of memory celltransistors M along the column direction are connected together toprovide an elongate control gate wiring layer CGj (j=1, 2, . . . , 8)shown in FIG. 3B. The resultant parallel control gate wiring layers CG1,CG2, . . . , CG8 constitute word lines of the EEPROM. Similarly, themutually-connected select gates 14-9, 16-9 of the first selecttransistor S1 constitute a select gate line that extends in the rowdirection; the mutually-connected select gates 14-10, 16-10 of thesecond select transistor S2 constitute another select gate lineextending in the row direction. The overall arrangement of the memorycell array section 1 including a plurality of similar NAND cell sectionsis illustrated in FIG. 4, wherein "Vs" is used to designate a commonsource potential of the EEPROM.

The detailed internal circuit configuration of the bit-line controller 2of FIG. 1 is shown in FIG. 5, wherein bit-line controller 2 includes aflip-flop circuit FF, which is arranged so that one flip-flop circuit isassociated with two adjacent ones (BL2i and BL2i+1) of the parallel bitlines BL. Flip-flop circuit FF is a circuitry that performs a data-latchfunction and a data sense/amplification function. Suchlatch/sense-amplifier circuit may perform different functions inresponse to control signals Vsn, Vsp so that it functions as a datalatch circuit during a designated time period, and alternatively servesas a sense amplifier circuit during the other time period.

The flip-flop circuit FF is a complementary MOS (CMOS) flip-flopcircuitry being comprised of enhancement-type (E-type) P-channel metaloxide semiconductor field effect transistors (MOSFETs) Qp1, Qp2 andE-type N-channel MOSFETs Qn5, Qn6, which are cross-coupled to MOSFETsQp1, Qp2 as shown in FIG. 5. CMOS flip-flop circuit FF has two circuitnodes N1, N2, which are coupled to bit lines BL2i (i=0, 1, 2, . . . ),SL2i+1 by way of E-type N-channel MOSFETs Qn7, Qn8, respectively.N-channel MOSFETs Qn7, Qn8 perform switching operations in response tocontrol signals φA, φB, respectively, thereby CMOS flip-flop circuit FFto be selectively connected to corresponding bit lines associatedtherewith or to be disconnected therefrom.

As shown in FIG. 5, the bit lines BL2i, BL2i+1 are provided withtransistor circuits respectively. A series circuit of E-type N-channelMOS transistors Qn9, Qn10 is coupled between the bit line and the powersupply voltage Vcc. A series circuit of E-type N-channel MOS transistorsQn11, Qn12 is coupled between the bit line BL2i+1 and the power supplyvoltage Vcc. The gate electrode of MOS transistor Qn10 is controlled bythe potential at one (N1) of the nodes N1, N2 of the CMOS flip-flopcircuit FF, the gate of MOS transistor Qn11 is controlled by the other(N2) thereof. The gates of the remaining MOS transistors Qn9, Qn12 aresupplied respectively with verify-read signals φAV, φBV, which go highduring a verify-read period. Either bit line BL2i or bit line BL2i+1 maybe potentially charged, by a corresponding transistor circuit associatedtherewith, toward a potential level that is less than the power supplyvoltage Vcc by the threshold voltage Vth of a corresponding E-typeN-channel transistor, i.e., Vcc-Vth. The bit lines BL2i, BL2i+1 are alsoprovided with E-type N-channel MOS transistors Qn13, Qn14 respectively.These transistors are precharging transistors responsive to prechargecontrol signals φPB, φPA, respectively.

E-type N-channel MOS transistors Qn3, Qn4 are provided for equalizingthe two nodes N1, N2 of the CMOS flip-flop circuit FF in response to anequalize control signal φE. E-type N-channel MOS transistors Qn1, Qn2are connected between the CMOS flip-flop circuit FF and a pair ofinput/output lines I/O, I/O. The transistors Qn1, Qn2 serve as transfergates that are responsive to a column select signal CSLi and causeflip-flop FF to be selectively coupled to the I/O line pair to perform adata input/output operation.

A voltage Vsw shown in FIG. 5 is the potential at an N type well regionin which the P-channel MOS transistors Qp1, Qp2 of the CMOS flip-flopcircuit FF are formed. The voltage Vsw is normally same in potential asthe power supply voltage Vcc; voltage Vsw is at an intermediate voltageVm (10 volts, for example) during a write period. A voltage Vsp to beapplied to the common source node of P-channel MOS transistors Qp1, Qp2remains at the potential level of power supply voltage Vcc. voltage Vspdrops down to the intermediate voltage Vm during the write period, andchanges temporarily at a different potential level equal to the half ofthe power supply voltage, i.e., Vcc/2 (=2.5 volts), during a readperiod. A voltage Vsn to be applied to the common source node of theN-channel MOS transistors Qn5, Qn6 is zero volts. The common sourcevoltage Vsn rises at the Vcc/2 temporarily. The potentials of prechargesignal Vsa, Vsb are as follows: When the bit line BL2i is selectedduring a read period, voltage Vsa is at about 3 volts, while voltage Vsbis Vcc/2. When the bit line BL2i+1 is selected during the read period,voltage Vsa is at Vcc/2, and voltage Vsb is 3 volts. During a writeperiod, the voltages Vsa, Vsb are at the intermediate potential Vm.These voltages are zero volts when the bit lines are reset after thecompletion of write and erase operations.

The operation of the EEPROM arranged as described above is as follows.When the EEPROM is set in a read mode, the signals φA, φB are at the low("L") level as shown in FIG. 6, thus causing the CMOS flip-flop circuitFF to be electrically disconnected from the bit lines BL. The prechargesignals φPA, φPB go high ("H" level) so that the bit lines areprecharged. Note that, with the present embodiment, the even-numberedones (even bit lines) BL2i of the bit lines BL are first selected duringthe first half of the read period as shown in FIG. 6, and that theodd-numbered ones (odd bit lines) BL2i+1 are then selected during thesecond half of the read period as shown in FIG. 7.

Applying the precharge signals φPA, φPB causes each of the even bitlines BL2i to be precharged up to 3 volts and causes each of the odd bitlines BL2i+1 to be precharged at zero volts. After the precharge iscompleted, voltage φPA drops at the "L" level, causing the even bit lineBL2i to be electrically floating. Thereafter, the row decoder 5generates a desired voltage, which is supplied to the select gates andthe control gates of the memory cell array section 1. For purposes ofexplanation, the second control gate CG2 is assumed to be selected fromamong the control gates CG of FIG. 4. Under such a condition, theselected control gate CG2 is at zero volts, the remaining control gatesCG1, CG3 to CG8 are at voltage Vcc with the first and second selectgates SG1, SG2 being set at Vcc. If the data stored in a selected memorycell transistor M2 is "1," the threshold voltage of it is positive, sothat no current flows. The potential on bit line BL2i remains at 3volts. Alternatively, if the cell storage data is "0," a cell currentbegins to flow, causing bit line BL2i to potentially drop down to 2.5volts or less.

All the control gates CG and the select gates SG1, SG2 are then reset tozero volts. The voltage Vsb is at Vcc/2 (=2.5 volts), causing bit lineBL2i+1 to be precharged to 2.5 volts. The CMOS flip-flop circuit FF isthen equalized when (1) a signal φE being supplied to the gates of MOStransistors Qp3, Qp4 of FIG. 5 is changed to the "H" level, and (2) thesignals Vsp, Vsn are at 2.5 volts. Thereafter, the signals φA, φB gohigh, causing CMOS flip-flop circuit FF to be connected to bit linesBL2i, BL2i+1. When Vsp is at Vcc, and when Vsn is zero volts, the bitline voltages are sensed differentially to provide a read data voltage,which is read externally and is latched continuously.

When the column select signal CSLi goes high, the read data is output tothe I/O lines, is transmitted to the data input/output (I/O) buffer 6,and is then taken out of it externally. Note that the operations in thecase of selection of the odd bit line BL2i+1 are similar to theaforementioned operations with the potential changes in the voltages φA,φPB and Vsa being replaced by φB, φPB and Vsb, respectively.

A write operation is performed as follows. The timing diagrams of FIGS.8 and 9 are pulsing sequences at the main components of the embodimentduring write, verify-read, rewrite, and verify-read operations,excluding a loading operation of the write data of the bit-linecontroller 2 from the data I/O buffer 6, under an assumption that theeven bit lines BL2i are selected. Before the execution of a writeoperation, all the memory cell transistors M are erased simultaneouslyat a time (block erase operation) in such a manner that a zero-voltvoltage is applied to the control gates of all the memory celltransistors M while the P type substrate (or both the N type substrateand the P type well region formed therein) is at the boosted high-levelvoltage Vpp (20 volts, for example). After the write data is suppliedfrom the data input/output buffer 6 to the CMOS flip-flop FF through theI/O lines and latched therein, the precharge signals φPA, φPB are set atthe intermediate potential Vm; then, the voltages Vsa, Vsb, Vsp, Vsw areat Vm. All the bit lines BL are at Vm-Vth. The two nodes N1, N2 of CMOSflip-flop FF are at either zero volts or Vm depending on the logic valueof the write data.

when the signal φA is changed at the intermediate potential Vm, the bitline BL2i changes in potential in accordance with the logic value ofwrite data: The bit line potential is Vm when the data is "0"; when thedata is "1," the bit line potential is at zero volts. Assume that thecontrol gate CG2 is selected by the row decoder 5 from the control gatesCG shown in FIG. 4. The selected control gate CG2 is set at Vpp, and theremaining control gates CG1, CG3 to CG8 are at Vm. At this time, thefirst select gate SG1 is at Vm, whereas the second select gate SG2 is atzero volts.

After a predetermined length of time has elapsed, the control gates CG1to CG8 and the select gate SG1 are reset to zero volts. The signal φA isthen at zero volts, causing the even bit line BL2i to be disconnectedfrom the CMOS flip-flop circuit FF. When the signals Vsa, Vsm drop downto zero volts, and when the signals φPA, φPB are at Vcc, all the bitlines BL are then reset to zero volts. The voltages Vsp, Vsw are at Vcc.

After the write operation, a read for write-verify (verify-read)operation is performed. The verify-read operation is basically similarto an ordinary read operation with a zero-volt voltage applied to aselected control gate being replaced by a 0.5-volt voltage, and with averify signal φAV being used. Firstly, the precharge signal φPA ischanged at 5 volts, causing bit line BL2i to be precharged to 3 volts.When precharge signal φPA is at the "L" level, bit line BL2i is renderedelectrically floating. The control gates and the select gates areselectively activated by the row decoder 5 so that the control gate CG2as being presently selected is at 0.5 volts, for example, and that thefirst select gate SG1 and the remaining control gates CG1, CG3 to CG8are at Vcc. While a "1" can be correctly read if the threshold voltageof memory cell transistors is higher than zero volts in a normal readmode, it may possibly occur in the verify-read mode that a "1" cannot beread unless the threshold voltage is 0.5 volts or more.

Thereafter, the voltage Vsb is at 2.5 volts (=Vcc/2), and the odd bitline BL2i+1 changes at 2.5 volts, accordingly. The even bit line BL2i isprecharged to the voltage Vcc-Vth in response to the verify signal φAV,if a logic "0" has been written. The equalize signal φE goes high. Thevoltages Vsp, Vsn are at 2.5 volts, causing the CMOS flip-flop circuitFF to be reset. With this reset, CMOS flip-flop circuit FF, which hasbeen functioning as a data-latch circuit, may "transform" to a senseamplifier. (A necessary data-latch may be performed by a parasiticcapacitance or stray capacitance that is inherently present in a wiringline coupled to the bit line.) Thereafter, the signals φA, φB are at the"H" level, causing nodes n1, N2 to be connected to bit lines BL2i,BL2i+1 respectively. The voltage Vsp is changed at Vcc, and the voltageVsn is at zero volts. A data on bit line BL2i is read out. The read datais then latched. The latched data will be used as a "rewrite" datalater. This rewrite data (data to be given to an insufficiently writtenmemory cell transistor if any) is converted from a memory cell dataobtained during the verify-read period on the basis of the write data aspreviously used. A conversion table for rewrite data is shown below.

                  TABLE 1                                                         ______________________________________                                        Write Data    1     1           0   0                                         Cell Data     0     1           0   1                                         Rewrite Data  1     0           0   0                                         ______________________________________                                    

The write operation including the verify-read and rewrite processes willbe terminated after a combination of the verify-read and the rewrite isrepeated for a predetermined number of times. The repeat value may beone hundred, for example. With such verify-read/rewrite scheme, after a"1" is written, if a data actually read out of the same memory celltransistor is "0," it should be determined that the cell transistor isan insufficiently written cell, which will be rewritten with the "1"data. In other words, whenever the threshold voltage of any once-writtencell transistor is not more than 0.5 volts, additional "1" writeoperation is performed therefor in order to raise the threshold voltage.After the execution of such addition writing or rewriting of the data"1," if the memory cell data is "1," a "0" is rewritten. This meansthat, if a memory cell transistor is found to have its threshold voltagehigher than 0.5 volts, a "0"-write is carried out with respect to suchcell transistor during the rewrite period, to thereby eliminate anyfurther increase in the threshold voltage of it. A "0"-rewrite follows a"0"-write, that is, a rewriting of "0" is automatically performed withrespect to any one of the memory cell transistors that have been writtenwith a "0." More specifically, only when the threshold voltage of a"1"-written cell transistor is lower than 0.5 volts, the "1" is writtenagain thereinto, thereby to enable any "1"-written cell transistor to besuppressed or prevented from being further increased undesirably.

The pulsing sequences of the control gates CG1 to CG8 and the selectgates SG1, SG2 during the erase, write, verify-read, read operations areindicated in Table 2 presented below. Note that the table contentsassume that the control gate CG2 is selected with the even bit line BL2ibeing selected. Also note that "FLT" is used to represent the floatingcondition.

                  TABLE 2                                                         ______________________________________                                                      Write                                                           Line        Erase   "1"    "0"   Read   Verify                                ______________________________________                                        Bit Line BL2i                                                                             FLT      0 V   10 V  3   V    3   V                               Bit Line BL2i+1                                                                           FLT     10 V   10 V  2.5 V    2.5 V                               Select Gate SG1                                                                           20 V    10 V   10 V  5   V    5   V                               Control Gate CG1                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG2                                                                          0 V     20 V   20 V  0   V    0.5 V                               Control Gate CG3                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG4                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG5                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG6                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG7                                                                          0 V     10 V   10 V  5   V    5   V                               Control Gate CG8                                                                          0 V     10 V   10 V  5   V    5   V                               Select Gate SG2                                                                           20 V     0 V    0 V  5   V    5   V                               Source Line FLT      0 V    0 V  0   V    0   V                               Substrate   20 V     0 V    0 V  0   V    0   V                               ______________________________________                                    

According to the EEPROM as described above, after a data is once writteninto each of the memory cell transistors M1 to M8 in the selected NANDcell section, the resultant threshold voltage of each cell transistor ischecked or verified by causing this cell transistor to be applied by thebit-line controller 2 with the verify voltage of a predeterminedpotential level (for example, an intermediate voltage between the powersupply voltage Vcc and the ground potential). If such verify testreveals the fact that a cell transistor or transistors are unsuccessfulin data write, in other words, if any memory cell transistor ortransistors whose threshold voltage remains unsuccessful in fallingwithin the allowable range of a target value even after the execution ofthe data writing operation is found out in the NAND cell section, such"insufficiently written" cell transistor is additionally written (orrewritten) with the same data. Thereafter, the verify operation isperformed again to verify whether or not the resultant threshold voltageis at the target level. The verify process will be repeated until thesatisfactory results are obtained wherein the threshold voltages of allthe memory cell transistors in the NAND cell section fall within theallowable range.

With the write-verify/rewrite scheme, it becomes possible, by repeatingthe write-verify and rewrite operations with the length of time requiredto effect a successful data write being shortened, to cause the memorycell transistors finally succeeded in data write to be limited in thedistribution of threshold voltage so that resultant threshold voltagesare all within the allowable range determined in accordance with thelogic value of write data. This enables the EEPROM to be maximized inthe performance and reliability of write operation.

Furthermore, according to the embodiment, the bit-line controller 2 isconstituted by a specific circuitry that changes its functionselectively or alternatively between the data latch and the senseamplification in response to the control signals Vsn, Vsp. The circuitryacts as a data latch circuit, and the same circuitry may function as asense amplifier at a different time. The data-latch/sense-amplifiercircuitry includes the CMOS flip-flop circuit FF, which is arranged incommon for adjacent ones BL2i, BL2i+1 of the parallel bit lines BL.Employing such flip-flop circuit arrangement can allow the NAND-cellEEPROM to be minimized in the number of additional circuit componentsthat will be required to achieve the write-verify/rewrite feature. Inother words, the embodiment can attain successful write-verify/rewriteoperations with its circuit scale being decreased to have substantiallythe same chip occupation area as that of a prior art having nowrite-verify function. Therefore, the EEPROM can attain an increasedintegration density.

In reduction to practice, the bit-line controller 2 of FIG. 5 may bemodified as shown in FIG. 10, wherein the CMOS flip-flop circuit FFconstituting the data-latch/sense-amplifier circuit of the presentinvention is arranged using a signal-synchronous CMOS inverter, whichincludes E-type P-channel MOSFETs Qp3, Qp4 and E-type N-channel MOSFETsQn17, Qn18, and another signal-synchronous CMOS inverter includingE-type P-channel MOS transistors Qp5, Qp6 and N-channel MOS transistorsQn19, Qn20. An N-channel MOS transistor Qn21 is connected between anoutput node of CMOS flip-flop circuit FF and bit line BLi. TransistorQn21 performs a switching operation in response to a control signal φFbeing applied to the gate electrode thereof, causing CMOS flip-flop FFand bit line BLi to be selectively connected to each other.

A series circuit of an E-type N-channel MOS transistor Qn22 and anE-type N-channel MOS transistor Qn23 is coupled between the bit line BLiand the power supply voltage Vcc. Transistor Qn22 is responsive to theoutput node of the CMOS flip-flop circuit FF; transistor Qn23 isresponsive to a control signal φV. These transistors are provided forprecharging bit line BLi toward the voltage Vcc-Vth in accordance with adata in CMOS flip-flop FF during a verify-read period.

An E-type P-channel MOS transistor Qp7 and a depression-type (D-type)N-channel MOS transistor Qd1 are connected in series to each other toconstitute a precharge circuit for precharging the bit line BLi to thepower supply voltage Vcc. In the precharge circuit, transistor Qd1 isprovided to prevent an undesirable high potential from being applied totransistor Qp7 during the erase operation and a write operation. AnE-type N-channel MOS transistor Qn24 is connected to bit line BLi at itsone current-carrying electrode. This transistor serves as a resettransistor that resets bit line BLi to zero volts. The two nodes of theCMOS flip-flop circuit FF are coupled to the I/O lines by way of E-typeN-channel MOS transistors Qn15, Qn16 respectively. These transistorsQn15, Qn16 are transfer gates that are responsive to the column selectsignal CSLi.

The bit-line controller shown in FIG. 10 operates as follows. Thepulsing sequences of the bit-line controller are shown in FIG. 11. Whenthe signal φF drops down to the "L" level, the bit line BLi iselectrically disconnected from the CMOS flip-flop circuit FF. Bit lineBLi is precharged to the power supply voltage Vcc when the prechargesignals φP, φP are at the "H" and "L" levels respectively. Thereafter,the row decoder 5 activates the first and second select gates SG1, SGand the control gates CG1 to CG8. Assuming that the control gate CG2 isselected, the selected control gate CG2 is set at zero volts, and theselect gates SG1, SG2 and the remaining control gates CG1, CG3 to CG8are applied with the power supply voltage Vcc. If the memory cell datais a logic "0," bit line BLi is at the "L" level. If the data is "1,"bit line BLi remains at the "H" level.

After the select gates SG and the control gates CG are reset to zerovolts, the signals φSP, φSP rise to the "H" level, and signals φSN, φRNdrop down to the "L" level, thus having the CMOS flip-flop circuit FFinactivated. The signal φF is then at "H" causing the resultant voltageon bit line BLi to be transferred to the output line of CMOS flip-flopFF. When φSP is at "L" and φSN is at "H," the potential on bit line BLiis sensed. The sensed potential is then latched when φRP is at "L" andφRN is at "H." The latched data is transferred to the I/O lines when thecolumn select signal CSLi goes high potentially.

Subsequently, a write/write-verify operation is carried out. The pulsingsequences during this period are illustrated in FIG. 12. After the writedata is latched in the CMOS flip-flop circuit FF from the I/O lines, theprecharge signal φP is at the "H" level, and precharge signal φP is atthe "L" level. The bit line BLi is thus precharged. A voltage Vmb, whichis applied to the P-channel MOS transistors Qp3, Qp5 of FIG. 10, ispotentially changed from the power supply voltage Vcc to theintermediate voltage Vm (=10 V). The signal φF is at Vm. The potentialon bit line BLi changes at either zero volts or Vm in accordance withthe logic value of the latched data. In the case of writing of a logic"1," bit line BLi is at zero volts; in the case of writing of a logic"0," bit line BLi is at Vm. At this time, the first select gate SG1 isat Vm, whereas the second select gate SG2 is at zero volts. Thepotentials on control gates CG are as follows: While the control gateCG2 is selected, the control gate CG1 is at Vm, the selected controlgate CG2 is at the high-level voltage Vpp (=20 volts), and the remainingcontrol gates CG3 to CG8 are at Vm.

When the select gates SG1, SG2 and the control gates CG1-CG8 are resetto zero volts, the signal φF drops to the "L" level, and the signal φRis at the "H" level, causing the bit line BLi to be reset to zero volts.This means that the EEPROM is on standby for the initiation of averify-read operation.

The following verify-read operation is essentially similar to anordinary read operation to be performed in the NAND-cell type EEPROM.Firstly, the precharge signal φP is changed at the "H" level, andprecharge signal φP is at the "L" level, thereby to precharge the bitline BLi toward the power supply voltage Vcc. The select gates SG andthe control gates CG are then driven by the row decoder 5 as will bedescribed below. After resetting these gates SG, CG, the verify signalφV goes high, so that the voltage Vcc-Vth appears on only one of the bitlines BL which has been written with a logic "0" data.

Thereafter, the signals φSP, φRP are at the "H" level. The signals φSN,φRN are at the "L" level. The signal φF is at the "H" level. When signalφSP is at "L" and signal φSN is at "H," the bit line potential issensed. Then, signal φRP is at "L" and signal φRN is at "H," causing therewrite data to be latched. The relation among the write data, thememory cell data and the rewrite data is same as shown in Table 1indicated previously.

The write/write-verify mode will be completed after the abovewrite/write-verify operations are repeated for one hundred times, forexample. The potential changes on the bit line BLi, the select gates SGand the control gates CG during the erase, write, read and verify-readperiods is shown in Table 3 presented below. Note here that Table 3assumes that the control gate CG2 is selected.

                  TABLE 3                                                         ______________________________________                                                      Write                                                           Line        Erase   "1"     "0"  Read  Verify                                 ______________________________________                                        Bit Line BLi                                                                              FLT      0 V    10 V 5 V   5    V                                 Select Gate SG1                                                                           20 V    10 V    10 V 5 V   5    V                                 Control Gate CG1                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG2                                                                          0 V     20 V    20 V 0 V   0.5  V                                 Control Gate CG3                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG4                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG5                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG6                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG7                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG8                                                                          0 V     10 V    10 V 5 V   5    V                                 Select Gate SG2                                                                           20 V     0 V     0 V 5 V   5    V                                 Source Line FLT      0 V     0 V 0 V   0    V                                 Substrate   20 V     0 V     0 V 0 V   0    V                                 ______________________________________                                    

Attention should be directed to the fact that, while the thresholdvoltage verifying operation uses a 0.5 volt-voltage as its referencevoltage in the above embodiments, the reference voltage may be modifiedin accordance with a possible threshold voltage distribution in thereduction to practice. Also, the time length assigned to the executionof one write operation may be modified as follows. If it is requiredmore strictly that the resultant distribution in the finally obtainedthreshold voltages of a number of memory cell transistors is exactlymaintained within a narrow range, the length of every write time shouldbe shortened to increase the repeat value of the write/verify operationsat shortened execution periods. In addition, while the above embodimentsare directed to the NAND-cell type EEPROMs employing the tunnelinjection effect, the present invention may also be applied to othertypes of NAND-cell EEPROMs, such as a "hot-electron" injecting NAND-cellEEPROM. The embodiment of FIG. 5 may be effective when the memory cellarray section 1 of FIG. 1 is modified to have what is called the "openbit line" configuration.

A NAND-cell EEPROM in accordance with a third embodiment of the presentinvention shown in FIG. 13 is similar to that of FIG. 1 with the memorycell section being divided into two separate cell blocks 1A, 1B. Thebit-line controller 2 is a common circuitry for the both cell blocks 1A,1B.

The bit-line controller 2 is arranged as shown in FIG. 14. This circuitconfiguration is basically similar to that shown in FIG. 5, except thefollowing modifications. The nodes N1, N2 of CMOS flip-flop circuitconstituting the data-latch/sense amplifier circuit are coupled to bitlines BLa of cell block 1A and those of cell block 1B by way oftransfer-gate MOS transistors Qn7, Qn8. In FIG. 14, one of the bit linesBLa in cell block 1A is illustrated as "BLai" (i=0, 1, 2, . . . ) forpurposes of illustration only. The same goes with the bit line BLbi(i=0, 1, 2, . . . ).

with the circuitry of FIG. 14, unlike that of FIG. 5, an N-channel MOStransistor Qn25 is arranged, as an activation transistor, at the commonsource node of the MOS transistors Qn5, Qn6 on the NMOS side of the CMOSflip-flop circuit FF. This transistor Qn25 is responsive to a clocksignal φF, which is applied to the gate of it. On the PMOS side offlip-flop circuit FF, a P-channel MOS transistor Qp8 is arranged at thecommon source node of the transistors Qp1, Qp2 as another activationtransistor.

The operation of the EEPROM will be described with reference to FIGS. 15to 18, wherein FIGS. 15 and 16 show the pulsing sequences of it during aread period, and FIGS. 17 and 18 illustrate the pulsing sequences duringa write/write-verify period.

During a read period, the signals φA, φB drop at the "L" level, causingthe bit lines BLai, BLbi to be electrically disconnected from the CMOSflip-flop circuit FF. When the precharge signals φPA, φPB go high, thebit lines are precharged. The example shown in FIGS. 15 and 16 assumesthat the bit line BLai of the first cell block 1A is first selected, andthat the bit line BLbi of the second cell block BLbi is then selected ina next read cycle. The first read cycle for bit line BLai will beexplained below.

As shown in FIG. 15, the selected bit line BLai is precharged to 3volts, and the non-selected bit line BLbi is precharged to 2 volts, inresponse to the precharge signals φPA, φPB. After the prechargeoperation is completed, signals φPA, φPB are at the "L" level. The bitlines BLai, BLbi are then rendered electrically floating. Thereafter,the row decoder 5 generates suitable voltages, which are supplied to theselect gates SG and the control gates CG. For purposes of explanation,assume that the control gate CG2 is selected presently. In this case,row decoder 5 provides control gate CG2 with a zero-volt voltage, andapplies the power supply voltage Vcc to the first and second selectgates SG1, SG2 and the remaining control gates CG1, CG3 to CG8. If alogic "1" data has been stored in a memory cell transistor associatedwith control gate CG2, no current flows because the threshold voltage ofsuch selected memory cell transistor is positive. The potential on bitline BLai thus remains at 3 volts. Alternatively, if the storage data isa logic "0," a corresponding cell current flows causing the potential onbit line BLai to decrease at 2 volts or less.

Subsequently, the signal φP rises to the "H" level, and the signal φN isat the "L" level. The signal φE is at the "H" level. The CMOS flip-flopcircuit FF is then equalized. After the equalizing of flip-flop FF,signals φA, φB are at the "H" level, causing the bit lines BLai, BLbi tobe electrically connected to flip-flop FF. When signals φP is at the "L"level, and when signal φN is at the "H" level, the bit line potential issensed differentially to provide a read data, which is then latched.When the column select signal SCLi changes to the "H" level, the readdata is output to the I/O lines, transferred to the data input/outputbuffer 6, and then taken out of it externally.

Thereafter, the bit line BLbi of the second cell block 1B is selectedfor a read. The read operation therefor is similar to that previouslyexplained, with the operations of signals φPB, φP being replaced by eachother, and φSA, φSB being replaced with each other.

When the EEPROM is set in a write mode, a write operation and awrite-read operation therefor are performed as shown in FIGS. 17 and 18,wherein a data load operation of loading the write data of the bit-linecontroller 2 from the input/output buffer 6 is not shown. Note that thepulsing sequences shown in FIGS. 17 and 18 are obtained when the bitline BLai of the first cell block 1A is selected. Prior to a data write,an erase (block erase) operation is performed with respect to all thememory cells at a time in such a manner that the control gates of allthe memory cell transistors M are set at zero volts, while thehigh-level voltage Vpp (=20 volts) is applied to the P type substrate(or the N type substrate and the P type well region formed therein).After the write data is transferred from the data input/output buffer 6to the CMOS flip-flop FF through the I/O lines and latched therein, theprecharge signals φPA, φPB are set at the "H" level, thereby to resetall the bit lines.

When the signals φA, Vsw are at the intermediate voltage Vm (=10 volts),the bit line BLai is potentially changed in accordance with the logicvalue of write data. If the data is a logic "0," the bit line voltage isat Vm; if the data is a logic "1," the bit line voltage is at zerovolts. Assuming that the control gate CG2 is selected, this control gateCG2 is set at zero volts, while the select gates SG and the remainingcontrol gates CG1, CG3 to CG8 are at Vm.

After the elapse of a predetermined length of time (40 microseconds, forexample), the select gates SG1, SG2 and the control gates CG1 to CG8 arereset, and then the signal φA is at zero volts. This disconnects the bitline BLai from the CMOS flip-flop circuit FF. When the signal φPA is atthe "H" level, all the bit lines are reset. The signal Vsw becomes atthe power supply voltage Vcc.

A read operation for a write-verify (write-verify read operation) isthen performed. Like the above-mentioned embodiment, a 0.5-volt voltagein stead of the 0-volt voltage is applied to the selected control gate,and the verify signal φAV is generated. The bit line BLai is prechargedto 3 volts; the bit line BLbi is precharged to 2 volts. Thereafter, theprecharge signals φPA, φPB drop to the "L" level, causing bit linesBLai, BLbi to be electrically floating. The select gates and the controlgates are selectively activated by the row decoder 5 such that theselected control gate CG2 is set at 0.5 volts, while the select gatesSG1, SG2 and the remaining control gates CG1, CG3 to CG8 are at thepower supply voltage Vcc. While a "1" may be read correctly if thethreshold voltage of memory cell transistors is higher than zero voltsin a normal read mode, it may possibly occur in the verify-read modethat a "1" cannot be read unless the threshold voltage is 0.5 volts ormore.

Thereafter, the bit line BLai is charged to the voltage Vcc-Vth inresponse to the verify signal φAV, if a logic "0" data has been written.The potential level of the precharge voltage performed by the verifysignal in this case may be higher than that of the selected bit line.After the CMOS flip-flop FF is reset in response to the equalize signalφE, the signals φA, φB rise at the "H" level, causing the nodes N1, N2to be coupled to bit lines BLai, BLbi respectively. The signal φPpotentially changes at the "L" level, whereas signal φN changes to the"H" level. A data voltage on bit line BLai is thus read out. The readdata is latched. This latched data will be used as a rewrite data. Thisembodiment is similar to the previous embodiment in data-conversionscheme wherein the rewrite data is converted from the memory cell dataobtained during the verify-read period in accordance with the write dataas shown in Table 1.

The write operation will be completed after the execution of thecombination of the verify-read and rewrite operations is repeated for apredetermined number of times; one hundred, for example. With suchverify-read/rewrite scheme of this embodiment, it is possible tosuppress or prevent any "1"-written memory cell transistor from beingincreased in threshold voltage unnecessarily.

The potential changes of the control gates CG1 to CG8 and the selectgates SG1, SG2 during the erase, write, verify-read and read operationsare indicated in Table 4 presented below. Note that the table contentsassume that the control gate CG2 and the bit line BLai are selected."FLT" means the floating condition.

                  TABLE 4                                                         ______________________________________                                                      Write                                                           Line        Erase   "1"     "0"  Read  Verify                                 ______________________________________                                        Bit Line BLai                                                                             FLT     10 V     0 V 3 V   3    V                                 Bit Line BLbi                                                                             FLT      0 V     0 V 2 V   2    V                                 Select Gate SG1                                                                           20 V    10 V    10 V 5 V   5    V                                 Control Gate CG1                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG2                                                                          0 V     20 V    20 V 0 V   0.5  V                                 Control Gate CG3                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG4                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG5                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG6                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG7                                                                          0 V     10 V    10 V 5 V   5    V                                 Control Gate CG8                                                                          0 V     10 V    10 V 5 V   5    V                                 Select Gate SG2                                                                           20 V     0 V     0 V 5 V   5    V                                 Source Line FLT      0 V     0 V 0 V   0    V                                 Substrate   20 V     0 V     0 V 0 V   0    V                                 ______________________________________                                    

The present invention may also be applied to NOR-cell type EEPROMs, onepreferred embodiment of which will be described below.

A NOR-cell EEPROM shown in FIG. 19 includes an array of memory cellseach consisting of one twin-gate MOS transistor M with a floating gateand a control gate. These memory cell transistors are arranged in rowsand columns to provide a matrix form. The rows of cell transistors M arecoupled to word lines WLi, WLi+1, WLi+2, . . . at their control gates.The columns of cell transistors M are coupled to bit lines BL0, BL1, . .. , BLm at the drain electrodes thereof. To reduce the threshold voltageof a cell transistor causing a logic "1" data to be written thereinto, aspecific voltage of the negative polarity (-12 volts, for example) isapplied to the control gate of that transistor, while the power supplyvoltage Vcc is applied to the drain of it. At this time, non-selectedcell transistors that have the control gates coupled in common to thecontrol gate of the selected cell transistor are applied with azero-volt voltage at their drains, thereby to cause these non-selectedcell transistors to remain unchanged in their threshold voltages.

As shown in FIG. 20, a bit-line controller 2 including thedata-latch/sense amplifier circuitry is connected to bit lines BLai,BLbi, which are associated with first and second NOR cell blocks 1A, 1Beach having the memory cell matrix of FIG. 19. Bit-line controller 2latches therein a data determining whether or not the threshold voltageof a selected cell transistor is to be changed. The data-latch/senseamplifier circuitry includes a CMOS flip-flop FF, which is similar tothat of FIG. 5. E-type N-channel MOS transistor Qn31, Qn32, Qn34, Qn35are verify-read devices. E-type N-channel MOS transistors Qn33, Qn36 arearranged for precharging and resetting the bit lines.

with the embodiment, after a threshold-voltage reduction operation isperformed with respect to a certain cell transistor, a resultantthreshold voltage is verified using a verify voltage having apredetermined potential level, which is applied to the control gate ofthe cell transistor that has been reduced in threshold voltage. If theverify operation reveals the fact that the resultant threshold voltageis not changed up to a target level, the same write operation isexecuted for the same cell transistor, thereby forcing the thresholdvoltage thereof to further decrease. The verify process will becontinued until it is verified that the threshold voltage of the celltransistor succeeds in falling within an allowable range.

The write and verify operations of the NOR-cell EEPROM will now bedescribed with reference to the timing diagram shown in FIG. 21. Priorto the execution of write operation, an erase operation is performed ina manner that those cell transistors associated with each word line areerased at a time. Such "word-by-word" erase operation is performed byapplying a boosted high-level voltage (20 volts, for example) to everyword line WLi coupled to the control gates of a corresponding row ofcell transistor, and by applying a zero-volt voltage to the bit linesBL. With such voltage application, electrons are injected into thefloating gates of these cell transistors, causing the threshold voltagesto increase beyond the power supply voltage Vcc.

The write operation is carried out for the memory cell transistorsconstituting one page. First of all, a precharge signal φPA drops to the"L" level, thus rendering bit line BLai electrically floating. A wordline WLaj of FIG. 20 is then at -12 volts. When φA is at the "H" level,bit line BLai is potentially changed according to the logic level of awrite data: when the write data is a logic "1" (releasing electrons fromthe floating gate), bit line BLai is at the power supply voltage Vcc;when the write data is a "0" (preventing electrons from being releasedfrom the floating gate), bit line BLai is at zero volts. After the wordline is reset, signal φPA goes high to reset the bit line. At this time,the write operation is terminated.

A verify operation is then performed. Signals Va, Vb being applied tothe precharging/resetting transistors Qn33, Qn36 of FIG. 20 are changedto about 3 volts, 5 volts, respectively, causing the bit line BLai to beprecharged to 3 volts, and causing bit line BLbi to be precharged at 2volts. Bit lines BLai, BLbi are rendered electrically floating whensignals φPA, φPB are returned to the "L" level. The word line potentialis equivalent to the verify voltage of 3.5 volts, so that a data is readout. If a logic "0" has been written in a memory cell transistor beingselected for a data read, bit line BLai remains at 3 volts. If a logic"1" has been written in the cell transistor being lower than 3.5 voltsin its threshold voltage, bit line BLai potentially decreases.

After the word line is at zero volts, the signal φAV is changed to the"H" level. When a logic "0" data is being latched to write the "0" intoa selected cell transistor, or when it is not required to further writea "1" data into a cell transistor that has already been written with"1," the two transistors Qn31, Qn32 of FIG. 20 are controlled to turnon, causing bit line BLai to potentially decrease below the potential ona corresponding dummy bit line, which is practically at zero volts. In acase where a logic "1" has been written into the selected celltransistor, bit line BLai does not vary in potential because transistorQn32 is rendered nonconductive.

Thereafter, the signals φP, φN are changed at Vcc/2. The signal φE goeshigh. The CMOS flip-flop FF is thus equalized. When φP is at the powersupply voltage Vcc, and when φN is at zero volts, a bit line potentialis read out, and is then latched as a rewrite data, which will be usedlater. The relation among the write data, the read data and the rewritedata is same as that of the embodiment previously explained withreference to Table 1.

Note here that, in the above embodiment, the memory cell matrix may beformed in a P type well region, which is applied with a negativevoltage, such as -12 volts, with the power supply voltage Vcc beingapplied to a selected word line, and the negative voltage (-12 volts)being also applied to the non-selected word lines. If it is requiredthat the definitions of "write" and "erase" are replaced by each other,the N-channel MOS transistors Qn31, Qn32, Qn34, Qn35 of FIG. 20 may beremoved to make narrower the threshold voltage distribution of thememory cell transistors in the same manner as in the embodimentexplained previously.

The internal circuit configuration of the row decoder 5 shown in FIG. 1or 13 may be arranged as shown in FIG. 22, wherein the row decoderincludes an enable circuit, which is constituted by E-type N-channel MOStransistors Qn41, Qn42 and E-type P-channel MOS transistors Qp11, Qp12,and a transfer circuit constituted by E-type N-channel MOS transistorsQn43, Qn44 and E-type P-channel MOS transistors Qp13, Qp14. The rowdecoder is activated by an address signal ai and a decoder-enable signalRDENB to select one of the cell blocks 1A, 1B shown in FIG. 13, 14 or20. The row decoder operates when a signal φER goes high, which isapplied to the common gate of transistors Qp11, Qn42 of FIG. 22. Avoltage VppRW being applied to the transistors Qp13, Qp14 is at thepower supply voltage Vcc during a read period; the voltage VppRW willrise at the boosted high-level voltage Vpp during an erase and a writeperiod.

E-type N-channel MOS transistors Qn50 to Qn69 and E-type P-channel MOStransistors Qp20 to Qp29 are arranged to constitute a plurality oftransfer gates, which may "open" selectively, in response to an outputsignal of the row decoder, thereby allowing (1) select-gate potentialsSG1d, SG2d, (2) control-gate potentials CG1D to CG8D and (3) thepotential at a non-selected select gate Vuss to be transferredtherethrough. The signals Vuss, SG1D, SG2D, CG1D to CG8D are common toeach row decoder.

The potential changes of the select gates SG1, SG2, the control gatesCG1 to CG8, the bit lines, and the signals SG1D, SG2D, CG1D-CG8D, Vuss,VppRW during the read, erase, write and verify-read operations areindicated in Table 5 below. The table contents assume that the controlgate CG2 and the bit line BLai are selected. "FLT" means the floatingcondition.

                  TABLE 5                                                         ______________________________________                                        Erase                                                                         Non-Select   Select  Write           Verify-                                  Line   Block     Block   "1"  "0"   Read Read                                 ______________________________________                                        BLai   FLT       FLT      0 V 10 V  3 V  3    V                               BLbi   FLT       FLT      0 V  0 V  2 V  2    V                               SG1    20 V      20 V    10 V 10 V  5 V  5    V                               CG1     0 V      20 V    10 V 10 V  5 V  5    V                               CG2     0 V      20 V    20 V 20 V  0 V  0.5  V                               CG3     0 V      20 V    10 V 10 V  5 V  5    V                               CG4     0 V      20 V    10 V 10 V  5 V  5    V                               CG5     0 V      20 V    10 V 10 V  5 V  5    V                               CG6     0 V      20 V    10 V 10 V  5 V  5    V                               CG7     0 V      20 V    10 V 10 V  5 V  5    V                               CG8     0 V      20 V    10 V 10 V  5 V  5    V                               SG2    20 V      20 V     0 V  0 V  5 V  5    V                               Source 20 V      20 V     0 V  0 V  0 V  0    V                               Substrate                                                                            20 V      20 V     0 V  0 V  0 V  0    V                               (P-well)                                                                      SG1D   20 V      20 V    10 V 10 V  5 V  5    V                               CG1D   20 V      20 V    10 V 10 V  5 V  5    V                               CG2D   20 V      20 V    20 V 20 V  0 V  0.5  V                               CG3D   20 V      20 V    10 V 10 V  5 V  5    V                               CG4D   20 V      20 V    10 V 10 V  5 V  5    V                               CG5D   20 V      20 V    10 V 10 V  5 V  5    V                               CG6D   20 V      20 V    10 V 10 V  5 V  5    V                               CG7D   20 V      20 V    10 V 10 V  5 V  5    V                               CG8D   20 V      20 V    10 V 10 V  5 V  5    V                               SG2D   20 V      20 V     0 V  0 V  5 V  5    V                               VppRW  20 V      20 V    20 V 20 V  5 V  5    V                               Vuss   20 V      20 V     0 V  0 V  0 V  0    V                               ______________________________________                                    

The above row decoder may alternatively be modified as shown in FIG. 23.The potential changes at the main components thereof are shown in Table6 presented below, wherein CG1D to CG8D are at zero volts rather than 20volts during an erase period.

                  TABLE 6                                                         ______________________________________                                        Erase                                                                         Non-Select   Select  Write           Verify-                                  Line   Block     Block   "1"  "0"   Read Read                                 ______________________________________                                        BLai   FLT       FLT      0 V 10 V  3 V  3    V                               BLbi   FLT       FLT      0 V  0 V  2 V  2    V                               SG1    20 V      20 V    10 V 10 V  5 V  5    V                               CG1    0 V       20 V    10 V 10 V  5 V  5    V                               CG2    0 V       20 V    20 V 20 V  0 V  0.5  V                               CG3    0 V       20 V    10 V 10 V  5 V  5    V                               CG4    0 V       20 V    10 V 10 V  5 V  5    V                               CG5    0 V       20 V    10 V 10 V  5 V  5    V                               CG6    0 V       20 V    10 V 10 V  5 V  5    V                               CG7    0 V       20 V    10 V 10 V  5 V  5    V                               CG8    0 V       20 V    10 V 10 V  5 V  5    V                               SG2    20 V      20 V     0 V  0 V  5 V  5    V                               Source 20 V      20 V     0 V  0 V  0 V  0    V                               Substrate                                                                            20 V      20 V     0 V  0 V  0 V  0    V                               (P-well)                                                                      SG1D   20 V      20 V    10 V 10 V  5 V  5    V                               CG1D   0 V        0 V    10 V 10 V  5 V  5    V                               CG2D   0 V        0 V    20 V 20 V  0 V  0.5  V                               CG3D   0 V        0 V    10 V 10 V  5 V  5    V                               CG4D   0 V        0 V    10 V 10 V  5 V  5    V                               CG5D   0 V        0 V    10 V 10 V  5 V  5    V                               CG6D   0 V        0 V    10 V 10 V  5 V  5    V                               CG7D   0 V        0 V    10 V 10 V  5 V  5    V                               CG8D   0 V        0 V    10 V 10 V  5 V  5    V                               SG2D   20 V      20 V     0 V  0 V  5 V  5    V                               VppRW  20 V      20 V    20 V 20 V  5 V  5    V                               Vuss   20 V      20 V     0 V  0 V  0 V  0    V                               ______________________________________                                    

A still another modification of the row decoder 5 is illustrated in FIG.24. This row decoder is similar to that of FIG. 23 with E-type P-channelMOS transistors Qp30 to Qp39 being added. While the potentials atnon-selected control gates are small as well as Vcc-Vh1 in thearrangement of FIG. 23, the row decoder arrangement of FIG. 23 will bemore preferable than that of FIG. 24 in that the former is less innecessary circuit area than the latter, if the potential Vh1 remainssmaller to the extent that the data storage characteristic of memorycell transistors is not affected practically.

The bit-line control circuit 2 shown in FIG. 5, 10 or 14 operates underthe power supply voltage Vcc of 5 volts. The bit-line controller will beused in a low-power EEPROM using a decreased power supply voltage of 3volts or less, which is fed by two dry batteries in a small computer,for example. If this is the case, the controller may be arranged as willbe explained below.

In the first place, assume that the N-channel MOS transistors Qn10, Qn11are less in threshold voltage than the other E-type N-channel MOStransistors. Such definition is introduced because the transmissionefficiency of voltage potential decreases if the threshold voltageremains higher. The recommended threshold voltage for an increasedvoltage transmission efficiency is as follows:

    Vcc-Vth(V.sub.RH)>VRH,

    Vth(V.sub.RL)>-VRL,

where, "V_(RH) " is the voltage on a bit line at the "H" level during aread period, and "V_(RL) " is a bit-line voltage at the "L" level duringthe read period. Assume that Vcc=3 V, V_(RH) =1.4 V, and V_(RL) =1.2 V.Vth(1.4)<1.6 V, and Vth(1.2)>-1.2 V, where the value in the parenthesis,such as 1.4, 1.2, indicates the back-bias voltage. The aforementionedother E-type N-channel MOS transistors increase in leak current if theirthreshold voltages Vth decrease; accordingly, Vth will not be modifiedin the practical application. By taking it into consideration, circuitsof FIGS. 25 and 26 are employed as the driver circuit for the signalsφPA, φPB, φAV, φBV, φA, φB, φE.

With the circuit of FIG. 25A, the "H" level of each signal can beincreased to become higher than the power supply voltage Vcc. Morespecifically, as shown in FIG. 25B, the gate of a D-type N-channel MOStransistor allowed Qd2 coupled between input and output voltages Vin,rout is potentially changed to zero volts after the elapse of a delaytime τ1 from a time point at which input signal Vin is equivalent toVcc. After a delay time τ2 has elapsed, output voltage rout is boostedto a potential level higher than Vcc.

A circuit shown in FIG. 26A is similar to that of FIG. 25A with ahigh-voltage switching circuit 10 being added. The high voltage may bethe intermediate voltage (=10 V) or the high-level voltage Vpp (=20/v).In this case, as shown in FIG. 26B, after delay time τ1 has elapsedsince input signals Vin3, Vin2 was changed at Vcc, the gate of theD-type N-channel MOS transistor Qd3 is set at zero volts. After theelapse of another delay time τ2, the output voltage Vcc is boosted tobecome higher than Vcc. An output operation of such high voltage is asfollows. As shown in FIG. 26C, after an input voltage Vin3 is at Vcc, asignal Vin1 is set at zero volts causing the gate of transistor Qd3 todrop to zero volts. Thereafter, when an input signal Vin4 supplied tohigh-voltage switcher 10 is set at Vcc, this circuit generates a highvoltage that potentially corresponds to either the intermediate voltageVm or the power supply voltage Vcc.

The pulsing sequences of the bit-line control circuit arranged describedabove are illustrated in FIGS. 27 to 30, which are similar to those ofFIGS. 15 to 18 with each signal being boosted as required.

A further embodiment for a Vcc-reduction of low-power EEPROMs will beexplained on the basis of the bit-line control circuit of FIG. 14,wherein the transistors Qn10, Qn11 are normally designed E-typeN-channel MOS transistors. The voltage Vsw for the CMOS flip-flopcircuit FF, which includes transistors Qn5, Qn6, Qn25, Qp1, Qp2, Qp8, isboosted up to an increased potential level during a verify-read period.The pulsing sequences of this embodiment are shown in FIG. 31, which issimilar to FIG. 30 with voltage Vsw being boosted.

FIGS. 32A to 32H show several possible modifications of the bit-linecontrol circuit 2 of the NAND-cell type EEPROMs previously described.Each of these figures indicates the circuit connected among thetransistors Qn9, Qn10 of FIG. 4, transistors Qn22, Qn23, transistorsQn9, Qn10 or Qn11, Qn12 shown in FIG. 14, the CMOS flip-flop FF and aselected bit line being associated therewith. The presentation teachesthat, even when N-channel MOS transistors are replaced by P-channel MOStransistors, the same circuit function may be attained by modifying therelated circuit-connection among transistors.

The bit-line control circuit 2 of FIG. 20 used in the NOR-cell typeEEPROM of FIG. 19 may be variously modified as shown in FIGS. 33A to33H, each of which indicates the connection between the transistorsQn31, Qn32 or Qn34, Qn35 of FIG. 20, the flip-flop circuit FF and aselected bit line associated therewith. In this case also, whenN-channel MOS transistors are replaced by P-channel MOS transistors, thesame circuit function may be attained by modifying the relatedconnection among transistors.

Note that, while the drain of the MOS transistor Qn9 is coupled to thepower supply voltage Vcc in the embodiment of FIG. 5, such gate voltagemay be changed to a different potential level that is higher than the"H" level potential of bit line during a read period. Similarly, thesource of the transistor Qn32 of FIG. 20, which is coupled to the groundpotential, may be changed to another potential that is lower than the"L" level of bit line during a read period.

With such an arrangement, the write/write-verify operation for suchEEPROM may be performed as shown in the flowchart of FIG. 34A or 34B. Inthe flowchart of FIG. 34A, a write data is input in a page mode. Afterthe data input, a write operation is carried out. A write-verify isexecuted with respect to every memory cell transistor that has beenwritten with the data, by reading the actual storage data of this celltransistor. If an output data is at a logic "1" level, that is, the celltransistor is an "insufficiently-written" cell transistor, a rewriteoperation is performed therefor. The set of the read for write-verifyprocess and the rewrite process will be repeatedly executed until anoutput data of each cell transistor is at a logic "1" level.

With the embodiment of FIG. 34B, the judgment of whether the output datais a logic "1" is performed with respect to all the memory celltransistor included being subjected to the data write at a time, afterthe write/write-verify processes have been repeated for a predeterminedtime (ten times, for example). The data read for a write-verify is notoutput every time. This will be advantageous in a case wherein the totalrepeat number of the write and verify-read processes, since the programexecution can become faster due to a decrease in the total length ofwrite period.

The present invention is not limited to the above-described specificembodiments and may be practiced or embodied in still other ways withoutdeparting from the spirit or essential character thereof.

What is claimed is:
 1. An electrically erasable and programmablenonvolatile semiconductor memory device, comprising:a plurality of bitlines; a plurality of word lines insulatively intersecting said bitlines; a memory cell array formed of electrically erasable andprogrammable memory cells coupled to said bit lines and said word linesand arranged in a matrix; a plurality of program circuits forcontrolling selection of memory cells, application of write voltages tothe selected memory cells and sensing of actual written states of saidselected memory cells; and plural data/verify circuits, coupled to saidmemory cell array and to said program circuits, for storing control dataat one of first and second predetermined logic levels which definewhether or not write voltages are to be applied to respective of thememory cells selected by said program circuits, for applying said writevoltages to said respective memory cells selected by said programcircuits in dependence on whether the control data corresponding to therespective of the memory cells is at said first pre-determined logiclevel or at said second predetermined logic level, for sensing actualwritten states of only given memory cells corresponding to data/verifycircuits in which control data of said first predetermined logic levelare stored, for modifying latest stored control data from said firstpredetermined logic level to said second predetermined logic level inthose data/verify circuits corresponding to memory cells in whichsuccessful writing of data has been sensed, for maintaining said storedcontrol data at said first predetermined logic level in the data/verifycircuits corresponding to memory cells in which it has been sensed thatdata has not been successfully written, and for maintaining said storedcontrol data at said second predetermined logic level in the data/verifycircuits storing said second predetermined logic level.
 2. The deviceaccording to claim 1, wherein said control data stored in said pluraldata/verify circuits are initially set to initial data, and then saidinitial data having said first predetermined logic level are modifiedwhen said plural data/verify circuits sense successful writing based onthe sensed actual written states of said given memory cells.
 3. Thedevice according to claim 2, wherein said initial data are loaded via atleast one input line.
 4. The device according to claim 3, furthercomprising at least one data buffer circuit for loading said initialdata into said plural data/verify circuits.
 5. The device according toclaim 1, wherein actual written states of said given memory cells aresimultaneously sensed by the data/verify circuits corresponding to saidgiven memory cells.
 6. The device according to claim 1, wherein saidplural data/verify circuits simultaneously sense actual written statesof said given memory cells.
 7. The device according to claim 1, furthercomprising bit line voltage regulators for selectively changing voltagesof said plurality of bit lines in accordance with said control datastored in said plural data/verify circuits.
 8. The device according toclaim 7, wherein said voltages of said plurality of bit lines areselectively and simultaneously changed by said bit line voltageregulators.
 9. The device according to claim 1, wherein a writeoperation and a verify operation are continued until all of saidselected memory cells are successfully written.
 10. The device accordingto claim 1, wherein said write voltages are simultaneously applied tosaid respective memory cells.
 11. The device according to claim 1,wherein said latest stored control data, in those data/verify circuitscorresponding to memory cells in which successful writing of data hasbeen sensed, are simultaneously modified from said first predeterminedlogic level to said second predetermined logic level.